Complement Factor D is part of the alternative pathway of the complement cascade and plays an early and central role in activating the pathway. Complement Factor C3b is generated from complement component C3 by the C3 convertase enzyme, distinct forms of which are produced following activation of the alternative pathway and the classical and lectin pathways. Activation of the alternative complement pathway is initiated by spontaneous hydrolysis of a thioester bond within C3 to produce C3(H2O), which associates with factor B to form the C3(H2O)B complex. Complement Factor D acts to cleave factor B within the C3(H2O)B complex to form Ba and Bb. The Bb fragment remains associated with C3(H2O) to form the alternative pathway C3 convertase C3(H2O)Bb. Additionally, C3b generated by any of the C3 convertases also associates with factor B to form C3bB, which factor D cleaves to generate the later stage alternative pathway C3 convertase C3bBb. This latter form of the alternative pathway C3 convertase may provide important downstream amplification within all three of the defined complement pathways, leading ultimately to the recruitment and assembly of additional factors in the complement cascade pathway, including the cleavage of C5 to C5a and C5b. C5b acts in the assembly of factors C6, C7, C8, and C9 into the membrane attack complex, which can destroy pathogenic cells by lysing the cell.
In some instances it is desirable to decrease the response of the alternative pathway. Activation of the alternative pathway of the complement cascade contributes to the production of C3a and C5a, both potent anaphylatoxins, which have roles in a number of inflammatory disorders. C3a and C5a are also formed by the classical pathway of the complement cascade. Regulation of the complement cascade is necessary to prevent damage to non-infected host cells. The balance between complement activation and inhibition is mediated by a series of complement regulatory factors, such as complement factor H and complement factor I.
Age-related macular degeneration (AMD) is a leading cause of vision loss in industrialized countries. There is evidence of the link between the complement cascade and macular degeneration. Individuals with mutations in the gene encoding complement factor H have a fivefold increased risk of macular degeneration and individuals with mutations in other complement factor genes also have an increased risk of AMD. Individuals with mutant factor H also have increased levels of C-reactive protein, a marker of inflammation. Without adequate functioning factor H the alternative pathway of the complement cascade is overly activated leading to cellular damage. Inhibition of the alternative pathway is thus desired.
Factor D is an attractive target for inhibition or regulation of the complement cascade due to its early and essential role in the alternative complement pathway, and its potential role in signal amplification within the classical and lectin complement pathways. Inhibition of Factor D effectively interrupts the pathway and attenuates the formation of the membrane attack complex. Compounds which act as Factor D inhibitors are needed for disruption of the complement cascade alternative pathway and treatment of disorders associated with defects in the complement cascade.